In the 1950s, the American government really enjoyed blowing up parts of Nevada. Operation Plumbbob was designed to test the ranges, effects, and variations among nuclear weapons: they detonated bombs underground, directly above observers, on missiles, in the middle of a whole bunch of pigs… one famous experiment put a nuclear bomb in an underground shaft with a huge metal plate above it.

I’ve seen some sources describe this plate as a manhole cover, but that’s not really accurate: it was a 900kg piece of steel plate armour. The test designer expected it to be shot upwards, so they set up a high-speed camera to capture its progress. It must have been fun to slow-motion through the footage: in one frame, the plate was resting on top of the shaft; in the next frame, it was in the air, impossibly high; in the next frame, it was gone.

They never found the plate. There are two possibilities: it was completely vaporised, or it was launched into orbit. Based on that one frame showing the plate in the air they could estimate its speed: at least 240,000 km/h, which is more than six times what it would need to escape Earth’s gravity. We’ll never know what happened to the plate, unless future generations find it orbiting the sun. If the plate left the planet, by the way, a solar orbit is its most likely endpoint.

240,000 km/h is 0.02% of the speed of light. It’s pretty fast. But we’ve since done better. The Helios B probe was launched in 1976 to study the sun. Its orbit is closer than Mercury, and because of its proximity it goes very fast. 252,792 km/h, fast enough to travel between New York and London in less than a minute and a half.

(Or between Johannesburg and Vladivostok in a little over three minutes. Or from Delhi to Sao Paulo in three and a half minutes.)

The new speed record will be set by a probe already in space: the Parker Solar Probe. Launched in 2018, it’s currently orbiting the sun and slingshotting past Venus to boost its speed and push it closer to the sun. It did its second Venutian fly-by just after Christmas last year; the next one is in July. By 2025 seven gravity assists from Venus will make it the fastest object ever made by human beings: it will be travelling at 690,000 km/h: 0.064% of the speed of light.